How to build a custom potentiometer?

Hello guys.
I'm trying to build a custom potentiometer by cutting a [nichrome resistance strip] to a U-shaped sheet (approximately 6cm * 15cm) and attaching a triangular metal part to check the resistance.
The triangular metal part will be attached to A0, and each end of the nichrome resistance strip will be attached to GND and 5V. the triangle is moved along the blue curve to change resistance sensed by A0.

I want the arduino to detect the triangle's movement based on the changing value of A0.

The problem is, I cannot decide on which nichrome resistance strip that I need to buy, because I don't know what the units which stand for resistivity exactly mean.

for example, in the given phrase
"Alloy Specific Resistance (at 20° Cµ Ω -m): 1.45+-0.07"

Could somebody please tell me what this means?

The product that I'm currently looking forward to buying is in the link below.

Is the wrong thing for making a pot out of. The resistance is way too low.
While your idea is theoretically possible there are some problems with the idea.

The first one is what happens to the signal into A0 when you are not touching the track? This would leave the input floating, so it could read anything from the last thing it read to either rail. It is no good simply putting a pull up or pull down resistor on it because that will give the same signal as the pot when the tap is at one end. You could mask off a portion of the end of the track so you never get the maximum.

As to the material you need to be aiming at something in the order of at least 1K to 100K, so you need to look at something like Velostat. Resistance measurements for sheet material are given in ohms per square. Per square what you may ask but the answer is per square anything, it is a unit less measure.

I built a musical instrument from Velostat, here is a video of it SpoonDuino

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The alloy resistance at 20 degrees C is 1.45 Ohms per meter, Plus or minus 0.07 Ohms. So you you use a meter of the wire, you can expect about 1.45 Ohms resistance.
Paul

I guess that is micro-Ohms per meter. So 1.45 thousands of an Ohm.

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Yes, low resistance is difficult to measure with the Arduino. Normally, low resistance is measured using the 4-wire method but you're still normally measuring a very-low voltage.

You show "10K" but that would require a very-very long wire! :stuck_out_tongue:

Depending on the wire gauge and length, putting 5V across a Nichrome wire isn't that much different from using copper wire. You'll create a "short" (or near-short), get a ton of current and possibly kill your power supply! :frowning:

Usually the specs give you Ohms-per-foot for whatever gauge you're buying. Or, here is an Ohms per foot chart. It looks like there are two kinds of Nichrome wire, but they are "close".

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When getting out a wire with a 10 kΩ by total length at using a wire with a cross section of 1 mm^2 with this material.

 10000 / 1.45 ≈ 6896.6

Therefore, the length of the U-shape of the illustration you wrote is about 7km.
Wow. It's a very BIG component! :wink:

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Jeez. That's obviously way to much.. It will take me years to spend that much materials. Thanks for your help sir!

Thanks for your help. If the arduino cannot detect any major differences, I'll need to figure out another plan:(

That's great! Do you have any suggestions other than velostat? That material is unavailable in our country:(

No, wrong in two ways. Resistivity is in ohm metres (as given!) and you ignored the µ

Its 1.45e-6 Ωm (ohms times metres).

resistance = resistivity x length / area
so
resistivity = resistance x area / length

thus units of resistivity are Ωm (ohm metres).

Yes nichrome is a metal alloy, very low resistivity - not quite as low as copper of course, but low.

Resistivity is an intrinsic property of a material, not dependent on the amount of material or its shape. Its like density and and specific heat capacity, and not like volume, mass or heat energy.

Perhaps you were thinking of linear resistivity of a wire, which is measured in ohm/m, and is not intrinsic.

Well it is packing material it should be.

You can use simply pencil marks, draw the shape and scribble to fill it in. Use a 2B pencil.

Carbon fibre is the obvious choice as its got more plausible resistivity and is hard-wearing. Tricky to cut to shape though.

I guess the OP has never seen a wire wound potentiometer or rheostat. Google for the word "rheostat". See how they are made using an insulating material for the core to hold the wire that is wound on the flat insulating material. If you want to use nichrome wire, get VERY tiny diameter and wind it and a thread together around the form. The thread will keep the adjacent nichrome from touching each other.
Paul

I’m not sure what your project us , but a good option is to use a mechanical linkage to drive a conventional or slide potentiometer .
Making you own reliable one , is not easy

Oddly enough I got some carbon fibre rods in various diameters for a project hoping for that sort of resistance, and when I measured it with a meter it was almost a dead short.

That is why they can build airplanes out of carbon fiber. Lightening strike can't tell it from aluminum.
Paul

Actually no, CF has significant resistance, enough to make it vaporize/explode on lightning strike, so they embed metal mesh as the surface to provide a low resistance route for the lightning.
For instance see: https://www.researchgate.net/publication/309231103_Lightning_Protection_of_Carbon_Fibre_Reinforced_Plastics_-An_Overview-

Interesting and makes sense. I guess the small planes made of CF in Central Oregon are not much concerned with lightening.
Paul

If flying VFR you shouldn't being going anywhere near a thunderstorm anyway.

Lightning is not the same as lightening :slight_smile: